Silicon oxide particles, making method, lithium ion secondary battery, and electrochemical capacitor

ABSTRACT

Silicon oxide particles each comprising an inner portion having an iron content of 10-1,000 ppm and an outer portion having an iron content of up to 30 ppm are suitable as negative electrode active material in nonaqueous electrolyte secondary batteries. Using a negative electrode comprising the silicon oxide particles as active material, a lithium ion secondary battery or electrochemical capacitor having a high capacity and improved cycle performance can be constructed.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Divisional of co-pending application Ser. No.13/894,570 filed on May 15, 2013, which claims priority under 35 U.S.C.§119(a) to Patent Application No. 2012-112405 filed in Japan on May 16,2012. All of the above applications are hereby expressly incorporated byreference into the present application.

TECHNICAL FIELD

This invention relates to silicon oxide particles for use as negativeelectrode active material in lithium ion secondary batteries andelectrochemical capacitors, a method of preparing the same, a lithiumion secondary battery, and an electrochemical capacitor.

BACKGROUND ART

In conjunction with the recent rapid advances of portable electronicequipment and communications instruments, nonaqueous electrolytesecondary batteries having a high energy density are strongly demandedfrom the aspects of cost, size and weight reductions. Approaches knownin the art to increase the capacity of such nonaqueous electrolytesecondary batteries include, for example, use as negative electrodematerial of oxides of B, Ti, V, Mn, Co, Fe, Ni, Cr, Nb, and Mo andcomposite oxides thereof (Patent Documents 1 and 2); application asnegative electrode material of wherein x≧50 at % and M=Ni, Fe, Co or Mnwhich is obtained by quenching from the melt (Patent Document 3); use asnegative electrode material of silicon oxide (Patent Document 4); anduse as negative electrode material of Si₂N₂O, Ge₂N₂O or Sn₂N₂O (PatentDocument 5).

Among others, silicon oxide is represented by SiO_(x) wherein x isslightly greater than the theoretical value of 1 due to oxide coating,and is found on X-ray diffractometry analysis to have the structure thatnano-size silicon ranging from several to several tens of nanometers isfinely dispersed in silica. Silicon oxide offers a greater batterycapacity than the currently available carbon by a factor of 5 or 6 on aweight basis and relatively good cycle performance due to a less volumeexpansion. For these reasons, batteries using silicon oxide as thenegative electrode material are regarded fully effective for use inportable electronic equipment such as mobile phones, lap-top computersand tablets. When the automotive application is considered, however,these batteries are insufficient in cycle performance and expensive.

CITATION LIST

Patent Document 1: JP 3008228

Patent Document 2: JP 3242751

Patent Document 3: JP 3846661

Patent Document 4: JP 2997741 (U.S. Pat. No. 5,395,711)

Patent Document 5: JP 3918311

DISCLOSURE OF INVENTION

As compared with the currently available carbonaceous active material,the silicon oxide-based active material is expensive and inferior incycle performance. A further improvement in battery performance of thesilicon oxide-based active material is desired. An object of theinvention is to provide silicon oxide particles which are improved incycle performance while maintaining the high battery capacity and lowvolume expansion of silicon oxide, so that the particles are effectiveas active material in negative electrode material for nonaqueoussecondary batteries; a method of preparing the silicon oxide particles;and a nonaqueous electrolyte secondary battery having a negativeelectrode using the silicon oxide particles.

With a focus on silicon oxide as a negative electrode active materialwhich surpasses the battery capacity of the currently available carbonmaterial, the inventors worked for a further improvement in batteryperformance and a cost reduction. It has been found that batteryperformance is affected by the distribution and content of iron insilicon oxide particles, that silicon oxide particles with betterproperties are relatively readily obtained by limiting the distributionand content of iron to a specific range, and that using such particulatesilicon oxide as negative electrode active material, a nonaqueouselectrolyte secondary battery having a high capacity and improved cycleperformance can be constructed.

In one aspect, the invention provides silicon oxide particles for use asnegative electrode active material in nonaqueous electrolyte secondarybatteries, each comprising an inner portion having an iron content of 10to 1,000 ppm and an outer portion having an iron content of up to 30ppm. Preferably, the silicon oxide particles have an average particlesize of 0.1 to 30 μm and a BET specific surface area of 0.5 to 30 m²/g.

In a second aspect, the invention provides a lithium ion secondarybattery comprising a positive electrode, a negative electrode, and alithium ion-conducting nonaqueous electrolyte, the negative electrodecomprising the silicon oxide particles defined above as negativeelectrode active material.

In a third aspect, the invention provides an electrochemical capacitorcomprising a positive electrode, a negative electrode, and a conductiveelectrolyte, said negative electrode comprising the silicon oxideparticles defined above as negative electrode active material.

In a fourth aspect, the invention provides a method for preparingsilicon oxide particles for use as negative electrode active material innon-aqueous electrolyte secondary batteries, comprising the steps ofproviding a feed material capable of generating SiO gas and having aniron content of 100 to 20,000 ppm, heating the feed material in an inertgas under normal or reduced pressure at a temperature in the range of1,100 to 1,600° C. to generate a SiO gas, cooling the gas fordeposition, and grinding the deposit in a grinding machine devoid ofiron contamination. Typically, the feed material is a mixture of asilicon dioxide powder and a metallic silicon powder.

ADVANTAGEOUS EFFECTS OF INVENTION

Using a negative electrode comprising the silicon oxide particles of theinvention as negative electrode active material, a lithium ion secondarybattery or electrochemical capacitor having a high capacity and improvedcycle performance can be constructed. The method of preparing siliconoxide particles is simple and lends itself to the manufacture on anindustrial scale. As a consequence, nonaqueous electrolyte secondarybatteries can be manufactured at low cost.

BRIED DESCRIPTION OF DRAWINGS

The only figure, FIG. 1 schematically illustrates a horizontal tubularfurnace used in the preparation of silicon oxide particles in oneembodiment of the invention.

DESCRIPTION OF EMBODIMENTS

As used herein, the term “ppm” is parts by weight per million parts byweight.

One embodiment of the invention is silicon oxide particles for use asnegative electrode active material in nonaqueous electrolyte secondarybatteries, wherein an iron content of the particle inner portion is 10to 1,000 ppm by weight, and an iron content of particle outer portion isup to 30 ppm by weight. Also contemplated herein is a negative electrodeactive material comprising the silicon oxide particles. A nonaqueouselectrolyte secondary battery using the silicon oxide particles asactive material in its negative electrode may be embodied as a lithiumion secondary battery or electrochemical capacitor having a highcapacity and improved cycle performance. Although the ground is not yetwell understood, it is presumed that the inclusion of iron in theparticle inner portion in a specific range causes a change to thecrystalline structure so that the volume change associated withocclusion and release of lithium ions is suppressed.

The iron content of the particle inner portion is 10 to 1,000 ppm. Ifthe inner iron content is less than 10 ppm, there is a tendency that theabove effect is not exerted and cycle performance is degraded.Inversely, if the inner iron content exceeds 1,000 ppm, which means thatthe content of iron as impurity is too high, the charge/dischargecapacity is reduced. Preferably the inner iron content is 20 to 800 ppm,more preferably 30 to 700 ppm.

The iron content of the particle outer portion is up to 30 ppm,preferably up to 20 ppm, and more preferably up to 10 ppm. Desirably,the outer iron content is as low as possible and even equal to 0 ppm. Aslong as the outer iron content is up to 30 ppm, a nonaqueous electrolytesecondary battery, typically lithium ion secondary battery, having anegative electrode using the particles as active material has aminimized probability of ignition accidents due to short-circuiting,leading to higher safety.

The iron contents of inner and outer portions of particles are valuesmeasured as follows.

(1) Total Fe Amount of Particles

The total Fe content of particles is measured by adding 50 wt % ofhydrofluoric acid to a powder sample. Once reaction begins, 50 wt % ofnitric acid is further added. The liquid is heated at 200° C. untilcomplete melting. The liquid is analyzed by ICP-AES (Agilent 730C).

(2) Fe Amount of Particle Outer Portion

A powder sample was combined with 50 wt % of aqua regia and heated at170° C. for 2 hours for dissolution. This is followed by cooling, staticholding, and filtration. The filtrate is analyzed by ICP-AES (Agilent730C).

Fe amount of particle inner portion=(total Fe amount)−(Fe amount ofouter portion)   (3)

From these amounts, the iron contents (weight basis) of inner and outerportions of particles are computed.

The silicon oxide particle powder should preferably have an averageparticle size of 0.1 to 30 μm, more preferably 0.2 to 20 μm. Setting theaverage particle size of silicon oxide powder to at least 0.1 μmprevents the powder from increasing its specific surface area toincrease a proportion of silicon dioxide on particle surface. Thisconcomitantly suppresses any reduction of a cell capacity when thepowder is used as active material in the negative electrode of anonaqueous electrolyte secondary battery. The setting also prevents thepowder from reducing its bulk density and hence, prevents any drop ofcharge/discharge capacity per unit volume. In addition, such siliconoxide powder is easy to prepare and a negative electrode may be easilyformed therefrom. Setting the average particle size of silicon oxidepowder to at most 30 μm prevents the powder from becoming foreignparticles when coated on an electrode and adversely affecting cellproperties. In addition, a negative electrode may be easily formed andthe risk of separation from the current collector (e.g., copper foil) isminimized. It is noted that the “average particle size” as used hereinis a particle diameter (median diameter) corresponding to a cumulativeweight of 50% in particle size distribution measurement by laser lightdiffractometry.

The silicon oxide particles should preferably have a BET specificsurface area of 0.5 to 30 m²/g, more preferably 1 to 20 m²/g. A surfacearea of at least 0.5 m²/g ensures a high surface activity and allows abinder to exhibit a bond strength during electrode fabrication, leadingto improved cycle performance upon repetition of charge/dischargecycles. A surface area of up to 30 m²/g is effective for preventing aproportion of silicon dioxide on particle surface from increasing toreduce the cell capacity when used as active material in a lithium ionsecondary battery negative electrode, suppressing any increase of theamount of solvent absorbed during electrode fabrication, and eliminatinga need to add a large amount of binder to maintain bond strength, with aconcomitant reduction of conductivity causing deterioration of cycleperformance. It is noted that the “BET specific surface area” as usedherein is a value measured by the BET single-point method of evaluatingan amount of N₂ gas adsorbed.

In one embodiment, the silicon oxide particles may be prepared byproviding a feed material capable of generating SiO gas and having aniron content of 100 to 20,000 ppm, heating the feed material in an inertgas under normal or reduced pressure at a temperature in the range of1,100 to 1,600° C. to generate a SiO gas, cooling the gas for effectingprecipitation or deposition, and grinding the deposit in a grindingmachine devoid of iron contamination.

The SiO gas-providing feed material having an iron content of 100 to20,000 ppm is not particularly limited as long as it is capable ofgenerating SiO gas. Most often, a mixture of a silicon dioxide (SiO₂)powder and a reducing powder is used. Examples of the reducing powderinclude metal silicon compounds and carbon-containing powders. Interalia, a metal silicon powder is preferably used because of higherreactivity and yield.

From a mixture of a silicon dioxide powder and a metal silicon powderwhich ensures a high reactivity and yield, SiO gas may be efficientlygenerated. By starting with such a powder mixture as the SiOgas-providing feed material, silicon oxide particles can be prepared ata high productivity, which particles may be used as negative electrodeactive material to construct a nonaqueous electrolyte secondary batterywith a high capacity and improved cycle performance. Additionally, iron(Fe) present in the feed material serves as catalyst, leading to animproved productivity and a reduction of preparation cost. Thus theinventive silicon oxide can be prepared in high yields. For the mixtureof a silicon dioxide powder and a metal silicon powder, any suitablemixing ratio may be selected. Preferably, the metal silicon powder andthe silicon dioxide powder are mixed in a molar ratio in the range:1<metal silicon powder/silicon dioxide powder<1.1, and more preferablyin the range: 1.01=metal silicon powder/silicon dioxide powder=1.08,when the presence of surface oxygen on the metal silicon powder andtrace oxygen in the reactor furnace is taken into account.

The silicon dioxide powder used herein should preferably have an averageparticle size of up to 0.1 μm, more preferably 0.005 to 0.1 μm, and evenmore preferably 0.005 to 0.08 μm. The metal silicon powder used hereinshould preferably have an average particle size of up to 30 μm, morepreferably 0.05 to 30 μm, and even more preferably 0.1 to 20 μm. If theaverage particle size of silicon dioxide powder exceeds 0.1 μm, or ifthe average particle size of metal silicon powder exceeds 30 μm, thenreactivity and/or productivity may decline.

It is critical that the iron content of the SiO gas-providing feedmaterial be 100 to 20,000 ppm. The iron content is preferably 200 to15,000 ppm, and more preferably 250 to 13,000 ppm. If the feed materialhas a Fe content of less than 100 ppm, the resulting silicon oxideparticles may have an inner portion Fe content of less than 10 ppm,which are outside the scope of the invention. If the feed material has aFe content in excess of 20,000 ppm, the resulting silicon oxideparticles may have an inner portion Fe content of more than 1,000 ppm,which are also outside the scope of the invention. The inclusion of ironin the feed material in the specific range is advantageous from theaspects of productivity improvement and cost reduction because Fe servesas catalyst to enhance reaction rate.

The iron content may be adjusted by any desired means, for example, byadding a certain amount of iron or an iron compound or by selectingiron-containing metallic silicon. Typically, it is simple to select anduse chemical grade metallic silicon.

In one embodiment, the SiO gas-providing feed material is heated in aninert gas under normal or reduced pressure at a temperature in the rangeof 1,100 to 1,600° C. to generate a SiO gas, and the gas is then cooledfor effecting precipitation or deposition, obtaining a precipitate ordeposit. A heating temperature below 1,100° C. is too low for reactionto proceed, leading to a reduced emission of SiO gas and hence, asubstantially reduced yield. If the heating temperature exceeds 1,600°C., problems arise that the feed is powder mixture can be melted tointerfere with reactivity and reduce the emission of SiO gas and achoice of the reactor material is difficult. For this reason, theheating temperature is in the range of 1,100 to 1,600° C. The presenceof an inert gas which may be under atmospheric or reduced pressure isessential during the heating step, because otherwise the SiO gas oncegenerated becomes unstable, and the reaction efficiency of silicon oxideis reduced, both leading to a reduced yield.

Upon cooling, the SiO gas precipitates as a deposit. The gas may becooled by any desired means, for example, by introducing the gas in acooling zone to deposit on a substrate, or by spraying the gas into acooling atmosphere. In one typical means, the mix gas flows in a coolingzone where the gas deposits on a substrate. Although the material of thesubstrate for deposition is not particularly limited, high-melting pointmetals such as stainless steel, molybdenum, tungsten and alloys thereofare preferably used for ease of working. The cooling zone is preferablyat a temperature of 500 to 1,000° C., more preferably 700 to 950° C. Adeposition temperature of at least 500° C. makes it easy to prevent thereaction product from increasing its BET surface area beyond 30 m²/g. Ifthe deposition temperature is equal to or lower than 1,000° C., a choiceof the substrate material is easy and the deposition apparatus may be oflow cost. The temperature of the deposition substrate may be controlledby heater power, thermal insulation ability (e.g., insulating wallthickness), forced cooling, or the like.

If necessary, the deposit may be ground to any desired particle size bywell-known means. Typically, the deposit is ground to the desiredparticle size by a grinding machine devoid of iron contamination. Asused herein, the “grinding machine devoid of iron contamination” is agrinding machine comprising a grinding section and a contact sectionboth made of an iron-free material. Although the iron-free material isnot particularly limited, preference is given to ceramic materialsincluding alumina, zirconia, SiAlON, silicon carbide, and siliconnitride based materials. By grinding the deposit to the desired particlesize on a grinding machine devoid of iron contamination, particles areobtained in which the iron content of particle outer portion is equal toor less than 30 ppm.

To impart electroconductivity to the resulting silicon oxide particles,carbon may be deposited or coated thereon by chemical vapor depositionor mechanical alloying. When carbon coating is employed, the coverage(or coating weight) of carbon is preferably 1 to 50% by weight, morepreferably 1 to 30% by weight based on the total weight of carbon-coatedsilicon oxide particles.

The chemical vapor deposition of carbon may be conducted by introducinga hydrocarbon base compound gas and/or vapor into a deposition reactorchamber at a temperature in the range of 600 to 1,200° C., preferably800 to 1,100° C. and under atmospheric or reduced pressure, wherethermal chemical vapor deposition takes place in a well-known manner. Itis also acceptable to form silicon composite particles in which asilicon carbide layer is formed at the silicon-carbon layer interface.The hydrocarbon base compound used herein is thermally decomposed at theindicated temperature to form carbon. Examples of the hydrocarbon basecompound include hydrocarbons such as methane, ethane, propane, butane,pentane, hexane, ethylene, propylene, butylene, and acetylene, alone orin admixture; alcohol compounds such as methanol and ethanol; mono- totri-cyclic aromatic hydrocarbons such as benzene, toluene, xylene,styrene, ethylbenzene, diphenylmethane, naphthalene, phenol, cresol,nitrobenzene, chlorobenzene, indene, coumarone, pyridine, anthracene,and phenanthrene, alone or in admixture, and mixtures of the foregoing.Also, gas light oil, creosote oil and anthracene oil obtained from thetar distillation step are useful as well as naphtha cracked tar oil,alone or in admixture.

The silicon oxide particles thus obtained are suitable as negativeelectrode active material for use in nonaqueous electrolyte secondarybatteries. Using the silicon oxide particles as active material, anegative electrode suitable for use in nonaqueous electrolyte secondarybatteries may be prepared. Since the nonaqueous electrolyte secondarybattery constructed using the negative electrode exerts good cycleperformance while maintaining a high battery capacity and a low volumeexpansion inherent to silicon oxide, it is best suited in the automotiveapplication where these properties are required.

In another aspect, the invention provides a lithium ion secondarybattery comprising a positive electrode, a negative electrode, and alithium ion-conducting nonaqueous electrolyte, the negative electrodecomprising the inventive silicon oxide particles as active material. Thelithium ion secondary battery using the inventive silicon oxideparticles as active material in its negative electrode exhibits goodbattery properties such as charge/discharge capacity and cycleperformance.

When a negative electrode is prepared from a negative electrode materialcomprising the inventive silicon oxide particles, a conductive agentsuch as graphite may be added to the negative electrode material. Thetype of conductive agent used herein is not particularly limited as longas it is an electronic conductive material which does not undergodecomposition or alteration in the battery. Illustrative conductiveagents include metals in powder or fiber form such as Al, Ti, Fe, Ni,Cu, Zn, Ag, Sn and Si, natural graphite, synthetic graphite, variouscoke powders, meso-phase carbon, vapor phase grown carbon fibers, pitchbase carbon fibers, PAN base carbon fibers, and graphite obtained byfiring various resins.

The negative electrode is prepared by combining the silicon oxideparticles with a binder such as polyimide resin or aromatic polyimideresin, a conductive agent as mentioned above and additives, kneadingthem in a solvent such as N-methylpyrrolidone or water to form apaste-like mix, and applying the mix in sheet form to a currentcollector. The current collector used herein may be a foil of anymaterial which is commonly used as the negative electrode currentcollector, for example, a copper or nickel foil while the thickness andsurface treatment thereof are not particularly limited. The method ofshaping or molding the mix into a sheet is not particularly limited, andany well-known method may be used.

The lithium ion secondary battery is characterized by the use of thenegative electrode material comprising the inventive silicon oxideparticles as active material while the materials of the positiveelectrode, electrolyte, and separator and the battery design may bewell-known ones and are not particularly limited. For example, thepositive electrode active material used herein may be selected fromtransition metal oxides such as LiCoO₂, LiNiO₂, LiMn₂O₄, V₂O₅, MnO₂,TiS₂ and MoS₂, and chalcogen compounds. The electrolytes used herein maybe lithium salts such as lithium perchlorate in nonaqueous solutionform. Examples of the nonaqueous solvent include propylene carbonate,ethylene carbonate, dimethoxyethane, γ-butyrolactone and2-methyltetrahydrofuran, alone or in admixture. Use may also be made ofother various nonaqueous electrolytes and solid electrolytes.

The separator disposed between positive and negative electrodes is notparticularly limited as long as it is stable to the electrolyte liquidand effectively retains the liquid. Often, porous sheets or non-wovenfabrics of polyolefins such as polyethylene and polypropylene,copolymers thereof, and aramide resins are used. They may be used as asingle layer or a laminate of multiple layers while they may be surfacecovered with a layer of ceramic material such as metal oxide. Porousglass or ceramic material may also be used.

In a further aspect, the invention provides an electrochemical capacitorcomprising a positive electrode, a negative electrode, and a conductiveelectrolyte, the negative electrode comprising the inventive siliconoxide particles as active material. The electrochemical capacitor usingthe inventive silicon oxide particles as active material in its negativeelectrode exhibits good capacitor properties such as charge/dischargecapacity and cycle performance. The electrochemical capacitor ischaracterized by the negative electrode comprising the silicon oxideactive material defined herein, while other materials such aselectrolyte and separator and capacitor design are not particularlylimited. Examples of the electrolyte used herein include nonaqueoussolutions of lithium salts such as lithium hexafluorophosphate, lithiumperchlorate, lithium borofluoride, and lithium hexafluoroarsenate.Exemplary nonaqueous solvents include propylene carbonate, ethylenecarbonate, dimethyl carbonate, diethyl carbonate, dimethoxyethane,γ-butyrolactone, and 2-methyltetrahydrofuran, alone or a combination oftwo or more. Other various nonaqueous electrolytes and solidelectrolytes may also be used.

EXAMPLE

Examples and Comparative Examples are given below for furtherillustrating the invention although the invention is not limitedthereto.

Example 1

Silicon oxide was produced using a horizontal tubular furnace as shownin FIG. 1. A reactor tube 4 of alumina having an inner diameter of 80 mmwas coupled with a heater 1 and a vacuum pump 5 and a substrate 3 wasdisposed therein. A feed material was prepared by mixing equimolaramounts of chemical grade metallic silicon powder having an averageparticle size of 5 μm and a Fe content of 2,000 ppm and fumed silicapowder having a BET specific surface area of 200 m²/g and a Fe contentof 0 ppm. The reactor tube 4 was charged with 50 g of the feed material2. Notably, the feed material had a Fe content of 640 ppm.

Then the reactor tube 4 was evacuated to a reduced pressure of 20 Pa orlower by the vacuum pump 5 while it was heated up to 1,400° C. at a rateof 300° C./hr by the heater 1. The tube 4 was held at the temperaturefor 3 hours. With the heater 1 turned off, the tube was cooled to roomtemperature.

On cooling, the gas precipitated on the substrate 3 as a black massdeposit. The deposit was recovered 38 g while 4.8 g of a reactionresidue was left (conversion degree 90.4%). A 30-g portion of thedeposit was dry ground in a 2-L ball mill of alumina, yielding siliconoxide particles. The silicon oxide particles obtained were measured foraverage particle size and BET specific surface area. The productionconditions are tabulated in Table 1 and the measurement results areshown in Table 2.

[Cell Test]

The silicon oxide particles were treated as follows before a battery wasconstructed using the particles as negative electrode active material.The battery was evaluated for performance.

First, the silicon oxide particles were combined with 45 wt % ofartificial graphite having an average particle size of 10 μm and 10 wt %of polyimide. Further N-methylpyrrolidone was added thereto to form aslurry. The slurry was coated onto a copper foil of 12 μm thick, driedat 80° C. for 1 hour, and pressure formed into an electrode by a rollerpress. The electrode was vacuum dried at 350° C. for 1 hour, and punchedinto a piece of 2 cm² which served as a negative electrode.

To evaluate the charge/discharge performance of the negative electrode,a test lithium ion secondary cell was constructed using a lithium foilas the counter electrode. The electrolyte used was a nonaqueouselectrolyte solution of lithium hexafluorophosphate in a 1/1 (by volume)mixture of ethylene carbonate and diethyl carbonate in a concentrationof 1 mol/liter. The separator used was a microporous polyethylene filmof 30 μm thick.

The lithium ion secondary cell thus constructed was allowed to standovernight at room temperature. Using a secondary cell charge/dischargetester (Nagano K.K.), a charge/discharge test was carried out on thecell. Charging was conducted with a constant current flow of 0.5 mA/cm²until the voltage of the test cell reached 0 V, and after reaching 0 V,continued with a reduced current flow so that the cell voltage was keptat 0 V, and terminated when the current flow decreased below 40 μA/cm².Discharging was conducted with a constant current flow of 0.5 mA/cm² andterminated when the cell voltage rose above 2.0 V, from which adischarge capacity was determined.

By repeating the above operation, the charge/discharge test was carriedout 50 cycles on the lithium ion secondary cell. The discharge capacitywas evaluated after 50 cycles. The results of the cell test are shown inTable 2.

Example 2

Silicon oxide particles were produced as in Example 1 aside from usingceramic grade metallic silicon powder having a Fe content of 400 ppm asthe metallic silicon powder. As in Example 1, particle physicalproperties and cell performance were evaluated. The productionconditions are tabulated in Table 1 and the measurement results shown inTable 2.

Example 3

Silicon oxide particles were produced as in Example 1 except that 5 wt %of iron powder under #325 was added to the equimolar mixture of chemicalgrade metallic silicon powder having a Fe content of 2,000 ppm and fumedsilica powder having a BET surface area of 200 m²/g and a Fe content of0 ppm. As in Example 1, particle physical properties and cellperformance were evaluated. The production conditions are tabulated inTable 1 and the measurement results shown in Table 2.

Example 4

A solution of 3 g of iron nitrate nonahydrate in 500 mL of methanol wasprepared. To the solution was added 100 g of fumed silica powder havinga BET surface area of 200 m²/g (used in Examples 1 to 3). With stirringand mixing, the fumed silica was treated for about 2 hours. The treatedliquid was filtered. On drying, there was obtained Fe-bearing fumedsilica powder. On analysis, the fumed silica powder had a Fe content of3,500 ppm.

Silicon oxide particles were produced as in Example 1 aside from usingthis fumed silica. As in Example 1, particle physical properties andcell performance were evaluated. The production conditions are tabulatedin Table 1 and the measurement results shown in Table 2.

Comparative Example 1

Silicon oxide particles were produced as in Example 1 except that themetallic silicon in the feed material was obtained by treating theceramic grade metallic silicon powder having a Fe content of 400 ppm(used in Example 2) with hydrochloric acid, washing with water,filtering and drying. As in Example 1, particle physical properties andcell performance were evaluated. The production conditions are tabulatedin Table 1 and the measurement results shown in Table 2.

Comparative Example 2

Silicon oxide particles were produced as in Example 1 except that themetallic silicon in the feed material was a semiconductor grade metallicsilicon powder having a Fe content of 0 ppm. As in Example 1, particlephysical properties and cell performance were evaluated. The productionconditions are tabulated in Table 1 and the measurement results shown inTable 2.

Comparative Example 3

Silicon oxide particles were produced as in Example 1 except that 10 wt% of iron powder under #325 was added to the equimolar mixture ofchemical grade metallic silicon powder having a Fe content of 2,000 ppmand fumed silica powder having a BET surface area of 200 m²/g and a Fecontent of 0 ppm. As in Example 1, particle physical properties and cellperformance were evaluated. The production conditions are tabulated inTable 1 and the measurement results shown in Table 2.

Comparative Example 4

Silicon oxide particles were produced as in Example 1 except that thedeposit obtained in Example 1 was dry ground in a 2-L ball mill of steelfor particle size tailoring. As in Example 1, particle physicalproperties and cell performance were evaluated. The productionconditions are tabulated in Table 1 and the measurement results shown inTable 2.

TABLE 1 Fe content Metallic silicon (ppm) powder of Fe content Siliconoxide powder Grinding Grade (ppm) powder mixture technique Example 1Chemical 2,000 fumed silica 640 ball mill (Fe = 0 ppm) of alumina 2Ceramic 400 fumed silica 130 ball mill (Fe = 0 ppm) of alumina 3Chemical + 52,000 fumed silica 16,500 ball mill Fe 5% (Fe = 0 ppm) ofalumina 4 Chemical 2,000 fumed silica 3,000 ball mill (Fe = 3,500 ppm)of alumina Comparative 1 Ceramic, 77 fumed silica 25 ball mill ExampleHCl treated (Fe = 0 ppm) of alumina 2 Semiconductor 0 fumed silica 0ball mill (Fe = 0 ppm) of alumina 3 Chemical + 102,000 fumed silica32,500 ball mill Fe 10% (Fe = 0 ppm) of alumina 4 Chemical 2,000 fumedsilica 640 ball mill (Fe = 0 ppm) of steel

TABLE 2 Physical properties Reaction of silicon oxide particles Initialcell 50-th cycle cell residue Average BET Outer Inner performanceperformance Conversion particle surface Fe Fe Charge Discharge ChargeCycle Amount degree size area content content capacity capacity capacityretentivity (g) (%) (μm) (m²/g) (ppm) (ppm) (mAh/g) (mAh/g) (mAh/g) (%)Example 1 4.8 90.4 5.2 6.1 5 73 1,320 1,010 980 97 2 5.1 89.8 5.3 6.3 415 1,320 1,010 980 97 3 0.5 99.0 5.3 5.7 5 850 1,290 980 950 97 4 1.597.0 5.3 6.0 5 160 1,310 1,000 970 97 Comparative 1 8.2 83.6 5.2 6.4 5 71,310 980 920 94 Example 2 10.5 79.0 5.2 6.5 4 0 1,300 970 900 93 3 0.599.0 5.3 5.8 4 1,800 1,250 930 900 97 4 4.8 90.4 5.2 6.3 150 850 1,3201,010 980 97

As shown in Table 2, the silicon oxide particles produced by the methodof Example 1 had an average particle size of 5.2 μm, a BET surface areaof 6.1 m²/g, an outer Fe content of 5 ppm, and an inner Fe content of 73ppm. The silicon oxide particles of Example 2 had an average particlesize of 5.3 μm, a BET surface area of 6.3 m²/g, an outer Fe content of 4ppm, and an inner Fe content of 15 ppm. The silicon oxide particles ofExample 3 had an average particle size of 5.3 μm, a BET surface area of5.7 m²/g, an outer Fe content of 5 ppm, and an inner Fe content of 850ppm. The silicon oxide particles of Example 4 had an average particlesize of 5.3 μm, a BET surface area of 6.0 m²/g, an outer Fe content of 5ppm, and an inner Fe content of 160 ppm.

In contrast, the silicon oxide particles of Comparative Example 1 had anaverage particle size of 5.2 μm, a BET surface area of 6.4 m²/g, anouter Fe content of 5 ppm, and an inner Fe content of 7 ppm. The siliconoxide particles of Comparative Example 2 had an average particle size of5.2 μm, a BET surface area of 6.5 m²/g, an outer Fe content of 4 ppm,and an inner Fe content of 0 ppm. The silicon oxide particles ofComparative Example 3 had an average particle size of 5.3 μm, a BETsurface area of 5.8 m²/g, an outer Fe content of 4 ppm, and an inner Fecontent of 1,800 ppm. The silicon oxide particles of Comparative Example4 had an average particle size of 5.2 μm, a BET surface area of 6.3m²/g, an outer Fe content of 150 ppm, and an inner Fe content of 850ppm.

As also shown in Table 2, the lithium ion secondary cell having anegative electrode made of a material using the silicon oxide particlesof Example 1 marked an initial charge capacity of 1,320 mAh/g, aninitial discharge capacity of 1,010 mAh/g, a 50-th cycle dischargecapacity of 980 mAh/g, and a cycle retentivity of 97% after 50 cycles.The cell had a high capacity and improved cycle performance.

The lithium ion secondary cell using the silicon oxide particles ofExample 2 marked an initial charge capacity of 1,320 mAh/g, an initialdischarge capacity of 1,010 mAh/g, a 50-th cycle discharge capacity of980 mAh/g, and a cycle retentivity of 97% after 50 cycles. The cell hada high capacity and improved cycle performance like Example 1.

The lithium ion secondary cell using the silicon oxide particles ofExample 3 marked an initial charge capacity of 1,290 mAh/g, an initialdischarge capacity of 980 mAh/g, a 50-th cycle discharge capacity of 950mAh/g, and a cycle retentivity of 97% after 50 cycles. The cell had ahigh capacity and improved cycle performance like Examples 1 and 2.

The lithium ion secondary cell using the silicon oxide particles ofExample 4 marked an initial charge capacity of 1,310 mAh/g, an initialdischarge capacity of 1,000 mAh/g, a 50-th cycle discharge capacity of970 mAh/g, and a cycle retentivity of 97% after 50 cycles. The cell hada high capacity and improved cycle performance like Examples 1 to 3.

In contrast, the lithium ion secondary cell using the silicon oxideparticles of Comparative Example 1 marked an initial charge capacity of1,310 mAh/g, an initial discharge capacity of 980 mAh/g, a 50-th cycledischarge capacity of 920 mAh/g, and a cycle retentivity of 94% after 50cycles. The cell showed inferior cycle performance due to a low inner Fecontent as compared with the use of silicon oxide particles of Examples1 to 4. In addition, since the feed material contained no iron, theconversion degree was as low as 83.6%, indicating poor reactivity.

The lithium ion secondary cell using the silicon oxide particles ofComparative Example 2 marked an initial charge capacity of 1,300 mAh/g,an initial discharge capacity of 970 mAh/g, a 50-th cycle dischargecapacity of 900 mAh/g, and a cycle retentivity of 93% after 50 cycles.The cell showed a low capacity as compared with the use of silicon oxideparticles of Examples 1 to 4. In addition, since the feed materialcontained no iron, the conversion degree was as low as 79.0%, indicatingpoor reactivity.

The lithium ion secondary cell using the silicon oxide particles ofComparative Example 3 marked an initial charge capacity of 1,250 mAh/g,an initial discharge capacity of 930 mAh/g, a 50-th cycle dischargecapacity of 900 mAh/g, and a cycle retentivity of 97% after 50 cycles.The cell showed a low capacity as compared with the use of silicon oxideparticles of Examples 1 to 4.

The lithium ion secondary cell using the silicon oxide particles ofComparative Example 4 marked an initial charge capacity of 1,320 mAh/g,an initial discharge capacity of 1,010 mAh/g, a 50-th cycle dischargecapacity of 980 mAh/g, and a cycle retentivity of 97% after 50 cycles.The cell had equivalent battery properties to the use of silicon oxideparticles of Examples 1 to 4. When the number of cell operating cyclesexceeded 300 cycles, a short-circuit fault occurred between electrodes,giving off white fumes. The cell could be no longer used. It wasdemonstrated that the lithium ion secondary cell of Comparative Example4 lacked safety for practical service.

Notably, the lithium ion secondary cells using the silicon oxideparticles of Examples 1 to 4 ensured continuous charge/dischargeoperation even over 300 cycles.

While the invention has been illustrated and described in typicalembodiments, it is not intended to be limited to the details shown,since various modifications and substitutions can be made withoutdeparting in any way from the spirit of the present invention. As such,further modifications and equivalents of the invention herein disclosedmay occur to persons skilled in the art using no more than routineexperimentation, and all such modifications and equivalents are believedto be within the spirit and scope of the invention as defined by thefollowing claims.

Japanese Patent Application No. 2012-112405 is incorporated herein byreference.

Although some preferred embodiments have been described, manymodifications and variations may be made thereto in light of the aboveteachings. It is therefore to be understood that the invention may bepracticed otherwise than as specifically described without departingfrom the scope of the appended claims.

1. A lithium ion secondary battery comprising a positive electrode, anegative electrode, and a lithium ion-conducting nonaqueous electrolyte,said negative electrode comprising a silicon oxide particles as negativeelectrode active material having an average particle size of 0.1 to 30μm and a BET specific surface area of 0.5 to 30 m²/g, wherein a firstiron content of inner portion of the silicon oxide particles is 15 to1,000 ppm, and a second iron content of outer portion of the siliconoxide particles is 4 to 10 ppm, and wherein the inner portion isdifferent from the outer portion.
 2. The lithium ion secondary batteryof claim 1, the second iron content of outer portion of the siliconoxide particles is determined by a method where a powder sample is mixedwith 50% by weight of aqua regia and heated at 170° C. for dissolution,followed by cooling, static holding, and filtration, and obtainedfiltrate is analyzed by ICP-AES to determine the second iron content ofouter portion of the silicon oxide particles, and the first iron contentof inner portion of the silicon oxide particles is determined by amethod, where a total iron content of particles is measured by adding50% by weight of hydrofluoric acid to a powder sample, once reactionbegins, 50% by weight of nitric acid is further added to obtain aliquid, the liquid is heated at 200° C. until complete melting, theliquid is analyzed by ICP-AES to determine the total iron content ofparticles, and then the first iron content of inner portion of thesilicon oxide particles is determined by subtracting the second ironamount of outer portion from the total amount ((the total ironcontent)−(the second iron content of outer portion of the silicon oxideparticles)).
 3. The lithium ion secondary battery of claim 1, whereinthe first iron content of inner portion of the silicon oxide particlesis 20 to 800 ppm.
 4. The lithium ion secondary battery of claim 1,wherein the first iron content of inner portion of the silicon oxideparticles is 15 to 850 ppm, and the second iron content of outer portionof the silicon oxide particles is 4 to 5 ppm.